Miniaturization of semiconductor elements has reduced power-supply voltage of ICs. However, circuit scale of the semiconductor elements has been increased, which leads to the increase in operating current. As for devices driven by batteries, operating current greatly varies since their circuits are controlled to be repeatedly activated and stopped to extend battery life.
A DC-DC converter for supplying power to such devices is required to make output voltage constant even when output current changes, which leads to the need to broaden as much as possible the range of output current which can keep the output voltage at a desired level.
As an important index to keep the output voltage of the DC-DC converter constant, speed of response to the change in the output current is used. That is, even when operating current of an IC serving as a load on the DC-DC converter suddenly changes, it is required to keep the output voltage at a desired voltage value by controlling the DC-DC converter at high speed.
A ripple control DC-DC converter is known as a DC-DC converter achieving a fast response to the variation in the output current. In the ripple control DC-DC converter, a switch drive signal is generated based on voltage ripples in a power stage, and switching is performed through self-excitation. Since there is no need to supply a clock signal from the outside, operation speed is not limited by clock frequency, which realizes a fast response to load variation.
When performing ripple control, it is general to input, into a differential input comparator, a ripple control signal and an error signal obtained by comparing the output voltage with reference voltage. However, there is a problem that great power consumption of the differential input comparator deteriorates conversion efficiency when the output current of the DC-DC converter is small (i.e., at the time of a light load.)